How do you find your way round a town you knew 40 years ago?
Not easy under any circumstances, and even in a place that made such a lasting impression. In practice, the challenge quickly became almost impossible.
The town in question - Nairobi - population 1975 circa 500,000, has become a huge city, population 2017 circa 4.5 million.
As I drove, well, crawled really, given the suffocating traffic, around Kenya's capital this week, I often felt disorientated. I found myself saying, again and again, 'I don't remember this road, or that tower block, or that hotel, or that flyover'.
The truth, of course, was that I didn't remember that road, or that tower block, and so much else, because they had not been there. There was nothing to remember.
READ MORE: In Kenya's Baringo county, police raid, burn and murder
Intellectually, I had been prepared for this obvious truth. The demographic facts speak for themselves. Population growth of 800 per cent makes for some dramatic changes. And yet, emotionally, it was still a shock.
Not only was so much new, so much old had disappeared. The city had not only expanded outwards and upwards, it had devoured much of its earlier versions of itself.
I strained to look for familiar roads, lined by trees and bungalows. In many places, they'd been wiped off the face of the earth.
Only Nairobi's topography, its hills and valleys, its dips and troughs, gave me clues as to where I was, and tormented me with hazy memories. I wanted to reach out, to capture the whole picture of how things were. But it's gone, forever.
I told a Kenyan of my generation of my difficulties. He laughed, and said, "Don't worry, you're not the only one. I've never left, and even I get lost these days. The building never stops".
The pace of Nairobi's transformation is accelerating. The economic improvement of the past 15 years has led to a boom in property prices and construction.
READ MORE: In Kenya's Kisumu, prayers for 'Baba' Odinga's presidency
The new skyscrapers, giant apartment blocks and malls have transformed Nairobi, but there's much more to come.
A friend sent me a link to an article, 'List of 13 Skyscrapers Under Construction That Will Define Nairobi's Skyline'. They include plans for the tallest building in Africa, currently a giant hole in the ground, but that will soon be 67 stories of concrete and steel.
Marooned between these new towers are a handful of the old colonial-era bungalows. Sitting on land worth millions of dollars, they are doomed.
I could blame the corruption and greed that makes it so easy for well-connected land-grabbers to break rules and cut corners. But in truth, in any society, the economic logic would be overwhelmingly on the developer's side.
I sat in a small Nairobi garden, talking to a Kenyan who has closely followed these changes, and tried to resist the worst of them.
Irungu Houghton is a fighter, and an eternal optimist, undeterred by death threats. "We've mobilised our neighbourhood, and we've had small victories," he said.
Irungu told me a nearby hotel had agreed to remove the ugly spiked fence from around its perimeter, and replace it with plants.